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From: Virendra <virendradhavale@y...>

Hi Virendra,
Any data packet, irrespective of whether it is sent by host or device, and 
again irrespective, whether it is zero/non-zero length packet should bear 
the CRC (16-bit) field.

Cuming now to ur question of what would be the crc for the zero length 
packet,
as u say, there would be no data on which the crc is to be calculated. 
Hence, (i hope u know), the crc registers are in their default values(preset 
to 1). And since, crc is normally negated and sent, it will be all zeroes. 
This is the raw crc data. however, this data needs to be bitstuffed and nrzi 
encoded.

As such, the host and the device have the same crc values when the crc is 
calculated on the same packet. This holds gud for the zero length packet 
also.

Hope this helps,

Regards,
Gayathri Seshadri.

>From: Virendra <virendradhavale@y...>
>Reply-To: virendradhavale@y...,   "Discussion list about free,open 
>source USB IP core" <usb@o...>
>To: usb@o...
>CC: uptikhe@h...
>Subject: [usb] CRC in zero ln. data pkt.???
>Date: Mon, 30 Aug 2004 22:52:52 -0700 (PDT)
>
>Hi,
>
>I am into designing of USB device controller. The problem that I am facing 
>is that of the CRC in zero length data packet. There are three querries as 
>such:
>
>1. Do we, as a USB device, send 16 bit CRC when we send a zero length data 
>packet in the status stage of the control write transfer performed by host?
>
>2. If this CRC is to be sent, then what would be those 16 bits? Because, 
>there is no data as such that goes into my CRC calculator.
>
>3. What would be the CRC when host sends a zero length data packet?
>
>Please help me out.
>
>Regards.
>
>Virendra
>
>
>
>
>
>
>
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